Control valve | EPFL Graph Search

A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level. In automatic control terminology, a control valve is termed a "final control element". Operation The opening or closing of automatic control valves is usually done by electrical, hydraulic or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening. Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically operated valves require additional cabling and switch gear, and hydraulically actuated valves required high pressure supply and return lines for the hydraulic fluid.

Actuator, valve actuator unit and valve

Fabio Beco Albuquerque, Herbert Shea

An actuator (22) comprises at least one actuator body (32) of dielectric elastomeric material and two electrodes (34, 36) being attached to opposite surfaces of the actuator body (32), respectively. At least one of the actuator body (32) and at least one of the electrodes (34, 36) is at least partly covered at its outer side with a protective layer (38a, 38b) of polymer material, wherein a mechanical stiffness of the protective layer (38a, 38b) is at least three times lower than a mechanical stiffness of the actuator body (32). Moreover, a valve actuator unit comprises such an actuator (22) and a housing, wherein the actuator (22) is arranged within the housing and the remainder of the housing is filled with a gas. Additionally, a valve comprises such a valve actuator unit and a valve element, wherein the valve element is movable by the valve actuator unit.

2022

Non-Linear Stability Analysis of a Reduced Scale Model Pump-Turbine at Off-Design Operation

Sébastien Alligné, François Avellan, Vlad Hasmatuchi, Christian Landry, Andres Müller, Steven Roth

Nowadays, the pump-storage power plants are a proven solution for storing electricity at large scale and offering flexibility to the power management. Therefore, the hydraulic machines are increasingly subject to off-design operation, start-up and shutdown sequences. However, the fast and frequent switching between pumping mode and generating mode presents technical challenges. In the present study, the reduced scale model of a low specific speed pump-turbine is investigated in generating mode at off-design conditions. The operation in the typical “S-shaped” curve of pump-turbine may become unstable and the machine may switch back and forth from generating mode to reverse pumping mode preventing the correct experimental survey of this part during the model testing. The instability has been solved by a testing procedure imposing a restriction of section and a control valve for being able to increase the energy losses. This procedure, commonly used in model testing of pump-turbines, significantly improves the stability of the machine and allows for the survey of the entire “S-curve”. The aim of the present investigation is to understand and explain the origin of the switch to reverse pumping mode. Thus, a hydro-acoustic test rig model was developed with the In-house EPFL SIMSEN software and a comparison between the systems with and without a restriction of section was studied. A numerical analysis indicates that the operating points of a pump-turbine system are defined by the solution of the equation relating the test rig characteristic and the energy-discharge characteristic of the hydraulic machine for a given rotational speed and a constant guide vanes opening. Furthermore, the addition of a restriction alters the curvature of the test rig characteristic and creates a new degree of freedom to achieve stable operating points in the “S-curve”. Finally, to ensure the stability of each operating points described by the numerical model, an eigenvalue study of the non-linear hydraulic system is necessary.

2011

Dynamic Modeling and Simulation of Two-phase Evaporators and Condensers in the context of a CO2 Based District Energy Network

Daniel Favrat, Samuel Henchoz, François Maréchal

A new type of district energy network has recently been proposed that is based on the use of CO2 as a heat transfer fluid. It uses the latent heat of vaporization, instead of sensible heat, to store and transfer heat. Previous studies have focused on demonstrating the potential of such a network in terms of energy efficiency and economy. In order to assess the technical feasibility of such a network, and to address some of the safety related issues, an analysis of the network’s dynamics is required. Such an analysis requires models to be developed for the network’s central plant, heating users’ substations, cooling users’ substations, as well as for the piping. Moreover these dynamic models should be developed in such a way that they can be calibrated with the help of a future experimental setup. In this context a dynamic model of a plate evaporator (also useable as a plate condenser) is presented, it is based on a one dimensional local formulation of the conservation laws for mass, momentum and energy. In this formulation, mass and energy in the components are allowed to vary, while non stationary terms in the momentum equation are neglected. On the water side, frictional pressure drops are computed with Churchill equation while heat transfer coefficient is computed with Gnielinski’s correlation. On the refrigerant side, a constant friction factor was implemented, while heat transfer coefficient is set to a reference value at reference conditions of mass flux and pressure. Variations of the heat transfer coefficient due to changes with respect to these two conditions are handled with exponents. These values are to be calibrated experimentally in a future work. The thermal capacity due to the mass of construction material was also accounted for in the model. In order to select an appropriate control scheme to be used in the experimental facility and, later on in a real network, the behavior of a cooling user substation including a control valve and an evaporator is discussed. A proportional integral controller showed its ability to stabilize the system to a desired degree of superheat at the evaporator outlet. Finally, perturbation rejection tests were made, and showed the ability of the system to operate under changing conditions of CO2 pressure and inlet enthalpy, as well as changes in the water mass flowrate, and inlet enthalpy.